Enstatite
Encyclopedia
Enstatite is the magnesium endmember of the pyroxene
silicate mineral series enstatite (MgSiO3) - ferrosilite (FeSiO3). The magnesium rich members of the solid solution
series are common rock-forming mineral
s found in igneous and metamorphic
rocks. The intermediate composition, (Mg,Fe)SiO3, has historically been known as hypersthene
, although this name has been formally abandoned and replaced by orthopyroxene. When determined petrographically
or chemically the composition is given as relative proportions of enstatite (En) and ferrosilite (Fs) (e.g., En80Fs20).
s are orthorhombic (space group
Pbca) although three polymorphs
are known. The high temperature, low pressure polymorphs are protoenstatite and protoferrosilite (also orthorhombic, space group Pbcn) while the low temperature forms, clinoenstatite and clinoferrosilite, are monoclinic (space group P21/c).
Weathered
enstatite with a small amount of iron takes on a submetallic luster
and a bronze-like color. This material is termed bronzite
, although it is more correctly called altered enstatite.
Bronzite and hypersthene were known long before enstatite, which was first described by G. A. Kenngott
in 1855.
An emerald-green variety of enstatite is called chrome-enstatite and is cut as a gemstone
. The green color is caused by traces of chromium
, hence the varietal name. In addition, bronzite is also sometimes used as a gemstone.
. They also have a prismatic cleavage
that is perfect in two directions at 90 degrees. Enstatite is white, gray, greenish, or brown in color; its hardness is 5–6 on the Mohs scale, and its specific gravity
is 3.2–3.3.
s and metamorphic rock
s. Magnesian orthopyroxene occurs in plutonic rocks such as gabbro
(norite) and diorite
. It may form small idiomorphic phenocrysts and also groundmass grains in volcanic rocks such as basalt
, andesite
, and dacite
.
Enstatite, close to En90Fs10 in composition, is an essential mineral in typical peridotite
and pyroxenite
of the Earth's mantle. Xenolith
s of peridotite are common in kimberlite
and in some basalt
. Measurements of the calcium
, aluminum, and chromium
contents of enstatite in these xenoliths have been crucial in reconstructing the depths from which the xenoliths were plucked by the ascending magmas.
Orthopyroxene is an important constituent of some metamorphic rocks such as granulite
. Orthopyroxene near pure enstatite in composition occurs in some metamorphosed serpentines. Large crystals, a foot in length and mostly altered to steatite, were found in 1874 in the apatite
veins traversing mica
-schist
and hornblende
-schist at the apatite mine of Kjörrestad, near Brevig in southern Norway
.
Enstatite is a common mineral in meteorites. Crystal
s have been found in stony and iron meteorite
s, including one that fell at Breitenhach in the Erzgebirge, Bohemia
. In some meteorites, together with olivine
it forms the bulk of the material; it can occur in small spherical masses, or chondrule
s, with an internal radiated structure.
. Enstatite is thought to be one of the early stages for the formation of crystalline silicates in space and many correlations have been noted between the occurrence of the mineral and the structure of the object around which it has been observed.
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
silicate mineral series enstatite (MgSiO3) - ferrosilite (FeSiO3). The magnesium rich members of the solid solution
Solid solution
A solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase...
series are common rock-forming mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s found in igneous and metamorphic
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
rocks. The intermediate composition, (Mg,Fe)SiO3, has historically been known as hypersthene
Hypersthene
Hypersthene is a common rock-forming inosilicate mineral belonging to the group of orthorhombic pyroxenes. Many references have formally abandoned this term, preferring to categorise this mineral as enstatite or ferrosilite. It is found in igneous and some metamorphic rocks as well as in stony and...
, although this name has been formally abandoned and replaced by orthopyroxene. When determined petrographically
Petrography
Petrography is a branch of petrology that focuses on detailed descriptions of rocks. Someone who studies petrography is called a petrographer. The mineral content and the textural relationships within the rock are described in detail. Petrographic descriptions start with the field notes at the...
or chemically the composition is given as relative proportions of enstatite (En) and ferrosilite (Fs) (e.g., En80Fs20).
Polymorphs and varieties
Most natural crystalCrystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
s are orthorhombic (space group
Space group
In mathematics and geometry, a space group is a symmetry group, usually for three dimensions, that divides space into discrete repeatable domains.In three dimensions, there are 219 unique types, or counted as 230 if chiral copies are considered distinct...
Pbca) although three polymorphs
Polymorphism (materials science)
Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...
are known. The high temperature, low pressure polymorphs are protoenstatite and protoferrosilite (also orthorhombic, space group Pbcn) while the low temperature forms, clinoenstatite and clinoferrosilite, are monoclinic (space group P21/c).
Weathered
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...
enstatite with a small amount of iron takes on a submetallic luster
Lustre (mineralogy)
Lustre is a description of the way light interacts with the surface of a crystal, rock, or mineral. The word lustre traces its origins back to the Latin word lux, meaning "light", and generally implies radiance, gloss, or brilliance....
and a bronze-like color. This material is termed bronzite
Bronzite
Bronzite is a member of the orthopyroxene group of minerals, belonging with enstatite and hypersthene to the orthorhombic series of the group...
, although it is more correctly called altered enstatite.
Bronzite and hypersthene were known long before enstatite, which was first described by G. A. Kenngott
Gustav Adolph Kenngott
Gustav Adolph Kenngott was a German mineralogist.He was born at Breslau. After being employed in the Hofmineralien-Cabinett at...
in 1855.
An emerald-green variety of enstatite is called chrome-enstatite and is cut as a gemstone
Gemstone
A gemstone or gem is a piece of mineral, which, in cut and polished form, is used to make jewelry or other adornments...
. The green color is caused by traces of chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
, hence the varietal name. In addition, bronzite is also sometimes used as a gemstone.
Identification
Enstatite and the other orthorhombic pyroxenes are distinguished from those of the monoclinic series by their optical characteristics, such as straight extinction, much weaker double refraction and stronger pleochroismPleochroism
Pleochroism is an optical phenomenon in which a substance appears to be different colors when observed at different angles with polarized light.- Background :Anisotropic crystals will have optical properties that vary with the direction of light...
. They also have a prismatic cleavage
Cleavage (crystal)
Cleavage, in mineralogy, is the tendency of crystalline materials to split along definite crystallographic structural planes. These planes of relative weakness are a result of the regular locations of atoms and ions in the crystal, which create smooth repeating surfaces that are visible both in the...
that is perfect in two directions at 90 degrees. Enstatite is white, gray, greenish, or brown in color; its hardness is 5–6 on the Mohs scale, and its specific gravity
Specific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...
is 3.2–3.3.
Occurrence
Isolated crystals are rare, but orthopyroxene is an essential constituent of various types of igneous rockIgneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
s and metamorphic rock
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
s. Magnesian orthopyroxene occurs in plutonic rocks such as gabbro
Gabbro
Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....
(norite) and diorite
Diorite
Diorite is a grey to dark grey intermediate intrusive igneous rock composed principally of plagioclase feldspar , biotite, hornblende, and/or pyroxene. It may contain small amounts of quartz, microcline and olivine. Zircon, apatite, sphene, magnetite, ilmenite and sulfides occur as accessory...
. It may form small idiomorphic phenocrysts and also groundmass grains in volcanic rocks such as basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
, andesite
Andesite
Andesite is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and dacite. The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende. Magnetite,...
, and dacite
Dacite
Dacite is an igneous, volcanic rock. It has an aphanitic to porphyritic texture and is intermediate in composition between andesite and rhyolite. The relative proportions of feldspars and quartz in dacite, and in many other volcanic rocks, are illustrated in the QAPF diagram...
.
Enstatite, close to En90Fs10 in composition, is an essential mineral in typical peridotite
Peridotite
A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...
and pyroxenite
Pyroxenite
Pyroxenite is an ultramafic igneous rock consisting essentially of minerals of the pyroxene group, such as augite and diopside, hypersthene, bronzite or enstatite. They are classified into clinopyroxenites, orthopyroxenites, and the websterites which contain both pyroxenes...
of the Earth's mantle. Xenolith
Xenolith
A xenolith is a rock fragment which becomes enveloped in a larger rock during the latter's development and hardening. In geology, the term xenolith is almost exclusively used to describe inclusions in igneous rock during magma emplacement and eruption...
s of peridotite are common in kimberlite
Kimberlite
Kimberlite is a type of potassic volcanic rock best known for sometimes containing diamonds. It is named after the town of Kimberley in South Africa, where the discovery of an diamond in 1871 spawned a diamond rush, eventually creating the Big Hole....
and in some basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
. Measurements of the calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
, aluminum, and chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
contents of enstatite in these xenoliths have been crucial in reconstructing the depths from which the xenoliths were plucked by the ascending magmas.
Orthopyroxene is an important constituent of some metamorphic rocks such as granulite
Granulite
Granulites are medium to coarse–grained metamorphic rocks that have experienced high temperature metamorphism, composed mainly of feldspars sometimes associated with quartz and anhydrous ferromagnesian minerals, with granoblastic texture and gneissose to massive structure...
. Orthopyroxene near pure enstatite in composition occurs in some metamorphosed serpentines. Large crystals, a foot in length and mostly altered to steatite, were found in 1874 in the apatite
Apatite
Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− ions, respectively, in the crystal...
veins traversing mica
Mica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
-schist
Schist
The schists constitute a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is...
and hornblende
Hornblende
Hornblende is a complex inosilicate series of minerals .It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole....
-schist at the apatite mine of Kjörrestad, near Brevig in southern Norway
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
.
Enstatite is a common mineral in meteorites. Crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
s have been found in stony and iron meteorite
Meteorite
A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...
s, including one that fell at Breitenhach in the Erzgebirge, Bohemia
Bohemia
Bohemia is a historical region in central Europe, occupying the western two-thirds of the traditional Czech Lands. It is located in the contemporary Czech Republic with its capital in Prague...
. In some meteorites, together with olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
it forms the bulk of the material; it can occur in small spherical masses, or chondrule
Chondrule
Chondrules are round grains found in chondrites. Chondrules form as molten or partially molten droplets in space before being accreted to their parent asteroids...
s, with an internal radiated structure.
Enstatite in Space
Enstatite is one of the few silicate minerals that have been observed in crystalline form outside our Solar System, particularly around evolved stars and Planetary Nebulae such as NGC 6302NGC 6302
NGC 6302 is a bipolar planetary nebula in the constellation Scorpius. The structure in the nebula is among the most complex ever observed in planetary nebulae...
. Enstatite is thought to be one of the early stages for the formation of crystalline silicates in space and many correlations have been noted between the occurrence of the mineral and the structure of the object around which it has been observed.